The present invention relates to the field of two-dimensional capacitive touch sensors including touch screens and touch pads and their associated sensor and controller chip. In particular, it relates to the specific design of the electrode patterns used to create a physical sensor suitable to sense the presence of one or more touching objects when the sensor is situated behind, or is embedded in, an insulating cover material that is non-uniform in its thickness.
A capacitive touch sensor can be generalised as one that uses a physical sensor element comprising an arrangement of electrically conductive electrodes extending over a touch sensitive area and a controller chip connected to the electrodes and operable to measure changes in the electrical capacitance of each of the electrodes or the mutual capacitance between combinations of the electrodes.
FIG. 1 shows principal components of a generic capacitive touchscreen. Item 100 represents the physical sensor element as a unitary item. In this example the sensor element is constructed from a substrate 103 that could be glass or plastic or some other insulating material and upon which is arranged an array of electrodes consisting of multiple laterally extending parallel electrodes, X-electrodes 101, and multiple vertically extending parallel electrodes, Y-electrodes 102, which in combination allow the position of a touch 109 to be determined. In some cases, each electrode has more detailed structure. The electrodes, which are sometimes called traces, are made of some electrically conductive material such as Indium Tin Oxide (ITO) or copper. The nature of the various materials depends on the chosen characteristics of the touch screen. For example, a touch screen may need to be transparent in which case ITO electrodes and a plastic substrate are common. On the other hand a touch pad, such as provided in lieu of a mouse in lap top computers is usually opaque and hence can use lower cost copper electrodes and an epoxy-glass-fibre substrate (e.g. FR4). Referring back to the figure, the electrodes are electrically connected via circuit conductors 104 to a controller chip 105 which is in turn connected to a host processing system 106 by means of a communication interface 107. The host 106 interrogates the controller chip 105 to recover the presence and coordinates of any touch or touches present on, or proximate to the sensor 103. In the example, a front cover (also referred to as a lens or panel) 108 is positioned in front of the sensor 103 and a single touch on the surface of the cover 108 is shown as 109. Note that the touch itself does not make direct galvanic connection to the sensor 103 or to the electrodes 102. Rather, the touch influences the electric fields 110 that the controller chip 105 generates using the electrodes 102. The touch influence causes a change in the capacitance of one or more electrodes which the controller chip can detect and measure. Using a suitable mathematical manipulation of the relative changes in the electrodes' capacitance, the controller chip 105 can approximate the touch position on the cover's surface as a relative XY coordinate 111. The host system can therefore use the controller chip to detect where a user is touching and hence take appropriate action, perhaps displaying a menu or activating some function.
There are many different material combinations and electrode configurations to allow creation of a capacitive touch screen and the example shown is just one.
A further important concept relates to the way the controller chip uses the electrodes of the sensor element to make its measurement. There are two important classes of controller in this regard.
A first class is known as a “self capacitance” style. Reference is made to FIG. 2. In this design of a capacitive sensor, the controller 201 will typically apply some electrical stimulus 202 to each electrode 203 which will cause an electric field to form around it 204. This field couples through the space around the electrode back to the controller chip via numerous conductive return paths that are part of the nearby circuitry 205, product housing 206, physical elements from the nearby surroundings 207 etc. so completing a capacitive circuit 209. The overall sum of return paths is typically referred to as the “free space return path” in an attempt to simplify an otherwise hard-to-visualize electric field distribution. The important point to realise is that the controller is only driving each electrode from a single explicit electrical terminal 208; the other terminal is the capacitive connection via this “free space return path”. The capacitance measured by the controller is the “self capacitance” of the electrode (and connected tracks) relative to free space (or Earth as it is sometimes called) i.e. the “self capacitance” of the electrode. Touching or approaching the electrode with a conductive element 210, such as a human finger, causes some of the field to couple via the finger through the connected body 213, through free space and back to the controller. This extra return path 211 is relatively strong for large objects (such as the human body) and so gives a stronger coupling of the electrode's field back to the controller; touching or approaching the electrode hence increases the self capacitance of the electrode. The controller senses this increase in capacitance. The increase is strongly proportional to the area 212 of the applied touch and is normally weakly proportional to the touching body's size (the latter typically offering quite a strong coupling and therefore not being the dominant term in the sum of series connected capacitances).
In a self capacitance sensor the electrodes are normally arranged as an orthogonal grid, with a first set of electrodes on one side of a substantially insulating substrate and the other set of electrodes on the opposite side of the substrate and oriented at nominally 90° to the first set. There are also structures where the grid is formed on a single side of the substrate and small conductive bridges are used to allow the two orthogonal sets of electrodes to cross each other without short circuiting. One set of electrodes is used to sense touch position in a first axis that we shall call “X” and the second set to sense the touch position in the second orthogonal axis that we shall call “Y”. An example is shown in FIG. 4 which is commonly referred to as “the diamond pattern” and is optimally suited to self capacitance XY sensors. In this figure can be seen a first set of electrodes 401 and a second set of electrodes 402 on two sides of a substantially insulating substrate 403. In this example the first set is used to resolve touch (408) position in the X direction 404 and the second set to resolve in the Y direction 405. This position determination can be more clearly understood by reference to the depiction of the relative changes in capacitance using two graphs 406 and 407. The position is computed at high resolution using an interpolation algorithm acting on the relative changes in capacitance from each set of electrodes. This allows use of far fewer electrodes than would be possible without interpolation.
In a self capacitance touch sensor, the controller can either drive each electrode in turn (sequential) or it can drive them all in parallel. In the former sequential case, any neighbouring electrodes are typically grounded by the controller to prevent them becoming touch sensitive when they are not being sensed (remembering that all nearby capacitive return paths will influence the measured value of the actively driven electrode). In the case of the parallel drive scheme, the nature of the stimulus applied to all the electrodes is typically the same so that the instantaneous voltage on each electrode is approximately the same. The drive to each electrode is electrically separate so that the controller can discriminate changes on each electrode individually, but the driving stimulus in terms of voltage or current versus time, is the same. In this way, each electrode has minimal influence on its neighbours (the electrode-to-electrode capacitance is non-zero but its influence is only “felt” by the controller if there is a voltage difference between the electrodes).
The second class of controller is known as a “mutual capacitance” style. Reference is made to FIG. 3. In this design of a capacitive sensor the controller 301 will sequentially stimulate each of an array of transmitter electrodes 302 that are coupled by virtue of their proximity to an array of receiver electrodes 303. The resulting electric field 304 is now directly coupled from the transmitter to each of the nearby receiver electrodes; the “free space” return path discussed above plays a negligible part in the overall coupling back to the controller chip when the sensor is not being touched. The area local to and centred on the intersection of a transmitter and a receiver electrode is typically referred to as a “node”. Now, on application or approach of a conductive element 305 such as a human finger, the electric field 304 is partly diverted to the touching object 305. An extra return path to the controller 301 is now established via the body 306 and “free-space” in a similar manner to that described above. However, because this extra return path acts to couple the diverted field directly to the controller chip 301, the amount of field coupled to the nearby receiver electrode 303 decreases. This is measured by the controller chip 301 as a decrease in the “mutual capacitance” between that particular transmitter electrode and receiver electrodes in the vicinity of the touch. The controller senses this decrease in capacitance of one or more nodes. The capacitance decrease is nominally proportional to the area 307 of the touch (although the change in capacitance does tend to saturate as the touch area increases beyond a certain size to completely cover the nodes directly under the touch) and weakly proportional to the size of the touching body (for reasons as described above). The capacitance decrease also reduces as the distance between the touch sensor electrodes and the touching object increases. This is in-line with the normal capacitance equation:C=(ε0*εr*A)/d 
As can be seen, the capacitance C is inversely proportional to distance, d.
In a mutual capacitance sensor the transmitter electrodes and receiver electrodes are normally arranged as an orthogonal grid, with the transmitter electrodes on one side of a substantially insulating substrate and the receiver electrodes on the opposite side of the substrate. This is shown in FIG. 3. It should be understood that discussion of a single unitary substrate does not preclude use of a multi-layer substrate which can sometimes be advantageous for cost, ease of fabrication or for other reasons. In FIG. 3 a first set of transmitter electrodes 302 is shown on one side of a substantially insulating substrate 308 and a second set of receiver electrodes 302 is arranged at nominally 90° to the transmitter electrodes on the other side of the substrate. There are also structures where the grid is formed on a single side of the substrate and small insulating bridges are used to allow the transmitter and receiver electrodes to cross each other without short circuiting.
For both classes of controller, in order to accurately sense the position of a touch, the controller needs a stable low noise measurement of the capacitances formed by the physical sensor (each of the nodes in the case of a mutual capacitance type controller or each of the electrodes in the case of a self-capacitance type controller). Best performance is achieved when a touch causes a large relative change in those capacitance proximate to the touch, a small or zero capacitance change in regions away from the touch and that all of the measurements are stable over the time during which they are measured. Of course, in a real world system there are many sources of electrical disturbance that will contaminate the measurements, making them fluctuate. It is also the case that the amount of capacitive change caused by touching is finite. The amount of capacitive change can generally be thought of the “signal” in the system, and the fluctuations in the measurements can be thought of as the “noise”. The ability of the overall system to accurately resolve the true touch position on the physical sensor depends on the overall system measurement quality which is known as the signal-to-noise ratio (SNR). It is a fundamental property of a position measurement system (of any type) that the ability of such a system to resolve position is proportional to the SNR of the underlying measurements. Hence, it is a goal of a touch sensing system to simultaneously maximise the “signal” and reduce the “noise”.
A second aspect of a touch system that is generally accepted in the industry to be important for good end-user acceptance is the overall “feel” of the system in terms of its sensitivity to touch and that its behaviour is consistent in this regard. A touch sensor is often regarded subjectively as “good” if a very light touch to the surface of the cover lens just causes a response by the system. The exact definition of a “light touch” is elusive and the experience will tend to vary somewhat from user to user, being dependant on their age, gender and digit size, amongst other physical traits. Some users also like to operate touch panels with the back of their finger nails. Equally, a touch sensor that is too sensitive, and tends to respond before the user feels that they have actually contacted the outer lens surface, will often be seen as annoyance as it can tend to give the feeling of responding to “unintended” touches.
For systems using touch sensors that operate from behind an insulating lens/panel of a substantially constant thickness, the SNR across the touch surface will be substantially constant too. Achieving a uniform touch feeling is fairly straightforward; typically the controller will monitor changes in the sensor's capacitances and will apply a simple threshold algorithm to such changes to detect if they are sufficiently strong to warrant the controller transitioning to a “detect confirmed” state and reporting computed X Y position data to the connected host system. As soon as the capacitance changes drop below this threshold (or perhaps a now reduced threshold so affecting an amount of “detect hysteresis”) then the controller will return to a “no detect” state and will block X Y coordinate reporting to the host. This “thresholding” is done in a way that is not linked to the actual XY position touched; that is, the threshold is the same at all places on the touch sensor's surface. By reducing the threshold the sensor can be made to feel more sensitive to touch and by increasing the threshold the opposite is true.
A particular challenge arises when the touch sensor is positioned behind a surface that varies substantially in thickness from point to point. An example might be the attractive aesthetic effect of making a 1-dimensional or 2-dimensional curved transparent “lens” on the front of a mobile device.
FIGS. 5A and 5B illustrate in perspective view examples of one- and two-dimensionally curved “lenses”. The pleasing smooth curving form of the lens is a positive styling advantage in some cases. Where it is also desired to place a touch sensor behind such a surface an immediate problem can be seen; the sensitivity will vary dramatically from place-to-place. A modified controller detection algorithm can of course take account of the approximate decoded XY position to dynamically adjust the required detection threshold from place-to-place. In effect, the controller uses a deliberately “over sensitive” (lowered) threshold to make an initial determination of XY position for a light touch (or even the approach of a finger prior to making physical contact) anywhere on the sensor and then using a look-up table or formulaic method, computes a secondary threshold that the capacitance change must exceed to enter further into the “detect confirmed” state thus enabling reports of coordinate data to the host. This way the controller can make the overall detection “experience” feel roughly uniform over the surface.
One issue that this adaptive method does not address is that at the thickest part of the lens the SNR of the underlying sensor to a touch is relatively poor by virtue of the increased distance from a touching finger to the sensor electrodes. Hence, computing accurate, low jitter positional data using a regular sensor electrode design is difficult, particularly in high electrical noise environments such as when a portable device incorporating a touch sensor is connected to an external electrical supply, such as to a noisy wall outlet power charger or a wireless charger. Changing the sensor electrode design to improve the SNR over the entire sensor area will also tend to render the outer edges too sensitive (where the lens is thin) and also somewhat prone to secondary effects such as extreme sensitivity to moisture or sweat.
WO 2011/142332 A1 discloses a design of mutual capacitance sensor of the kind shown in FIG. 3 for use with curved lenses as shown in FIGS. 5A and 5B in which, to compensate for varying thickness over the sensor area, the overlap area of the transmitter and receiver electrodes at their sensing nodes (as considered in plan view) is made smaller where the lens is thicker.
WO 2011/142333 A1 discloses a design of self capacitance sensor of the kind shown in FIG. 4 for use with curved lenses as shown in FIGS. 5A and 5B in which, to compensate for varying thickness over the sensor area, the overlap area of the X and Y electrodes at their sensing nodes (as considered in plan view) is made larger where the lens is thicker.
The prior art solutions of WO 2011/142332 A1 and WO 2011/142333 A1 respectively for mutual and self capacitance sensors thus have in common that they vary the node overlap area on the upper and lower sides of the lens to compensate for variation in lens thickness across the sensor area, but differ in that the overlap area is varied in opposite senses.